Map.cast does not check for narrower key type on lookup

[davidmorgan]

It's not clear to me from the docs what the intended behaviour is

https://api.dart.dev/stable/2.10.4/dart-core/Map/cast.html

but I found it surprising that

{'one': 1}.cast<int, int>()['one']

succeeds in looking up a String key in a Map<int, int>; I was expecting it to either throw or return nothing.

[lrhn]

I think the documentation is saying that the lookup should throw. Could be read differently though.
It doesn't expose the key, but it does expose that the key exists.

The current behavior was likely deliberate. Since [] has argument type Object? there is no problem passing the string in, and no problem forwarding it to the original wrapped map. It's just that you'd expect a Map<int, int> to return null when a string is passed in. Returning null isn't throwing, and cast is defined in terms of throwing on invalid arguments ... so arguably it's working as intended.

I can't find a better behavior (don't want to throw a type error when passing something to an Object? parameter, don't want to return null when the map has a value, nothing good left then).

So, not planning to make any change.

[davidmorgan]

Would it be worth clarifying in the docs?

Specifically

If any operation exposes a non-RK key or non-RV value, the operation will throw instead.

could be amended to cover this case, e.g.

If any operation exposes a non-RK key or non-RV value, the operation will throw instead. Map lookup is an exception to this: because operator[] takes Object?, a non-RK key is allowed to return an RV value if one is present in the underlying Map.

Thanks.

[lrhn]

I'll look into documenting that methods accepting Object? do not check their argument, and passes it through unchanged.
This applies to multiple members across the different collections.

[davidmorgan]

Sounds good, thanks.